1. Field of the Art
This invention relates to a sampling device for a rapid and accurate determination of hydrogen in molten metal.
2. Description of the Prior Art
The hydrogen contained in a metallic material has a significant influence upon the properties of the metal. By way of example, a high concentration of hydrogen in a steel-making material not only leads to brittleness but can be a cause of serious defects such as white spots and fractures. Therefore, it is of vital importance that the behavior of hydrogen be accurately monitored throughout the melting process, e.g. in molten iron and molten steel, so that the hydrogen content of the material will be controlled within an appropriate range. This requirement will be met only if a procedure is established for an accurate determination of hydrogen in molten metal. To this end, it is essential that a truly representative sample be obtained from the molten metal bath to be analyzed without losses or pick-up of hydrogen. Various samplers have heretofore been proposed to meet the above requirement but none of the devices thus far developed is fully satisfactory.
The prior art sampling techniques applicable to molten metal, particularly to molten steel, may be classified into the following two categories, as now discussed below.
The a first method comprises quenching the sample at a sufficiently rapid rate to "freeze in" in the hydrogen in the molten steel within the sample.
Specifically, (1) a spoon-mould process, (2) an aspiration process (3) an evacuated quartz-tube process, etc. may be mentioned.
These processes excel in workability and, hence, are in widespread use but unless the quenching operation is performed with sufficient efficiency, a loss of hydrogen takes place, which inevidatably precludes a complete trapping of the hydrogen. The result is that these procedures yield fairly lower values than the true value although the magnitude of error depends on the level of hydrogen and the type of alloy.
A second method involves the provision of a reservoir in the sampling device for the hydrogen that is released during the quenching and solidification of the sample within the sampler. Specificially, the vacuum sampling as is explained in papers of 19th committee of "NIPPON GAKUJUTSU SHIKOOKAI", (H. Freichtinger method and other procedures are known and, in theory, ought to give true hydrogen values. By these procedures, however, the gaseous hydrogen and the residual hydrogen in the solidified specimen must be independently determined. This not only means that the workability is low but also implies increased chances of an analytical error. Moreover, the sample must of necessity be of complicated construction and the chance of success in obtaining a sample is low. Thus, apparently these techniques have not been employed commonly in practical operations.
Asise from the above procedures, the technique called the immersion mould or J. G. Bassett method was proposed as disclosed in "The Determination of Gases in Metals, The Iron and Steel Institute (1960)", at page 12. This method involves the use of a device, which, as illustrated in FIG. 1 (A), comprises a quartz tube 1, a copper mould 2 as fitted into the tube 1, a sealing means 3 and a thin-walled portion 5. If necessary, the internal cavity 4 of the mould is evacuated. With this device, the molten metal breaks through the thin-walled portion 5 into the cavity 4 of the mould. Since, by this method, it is no longer necessary to take a sample with a spoon, the loss of hydrogen at the time of sampling is reduced. Moreover, because the mould 2 is made of copper, the quenching effect on molten metal is high. The quenched and solidified metal is a removed from the resultant specimen is analyzed for hydrogen. While the loss of hydrogen at sampling is, therefore, low, the hydrogen released in the process of quenching is not measured. Thus, the method is still not free from the disadvantages that the values are lower then the true hydrogen contents, ant that the quenching effect is not sufficient. In the taking of a sample from a molten metal for analysis, the supersaturating hydrogen is released as the result of the reduced solubility of hydrogen due to a sharp reduction in temperature of the specimen. Therefore, the loss of hydrogen is inevitable in the sampling stage and, in the above-described prior art methods, this loss of hydrogen has to be practically disregarded.
Illustrated in FIG. 1 (B) is a sampler which is most commonly utilized today. The reference numeral 1 indicates a quartz tube having a thin-walled portion 5. A substantial vacuum is maintained within the quartz tube 1 and, when the molten metal is sampled, the portion of the specimen obtained in a central part of the sampler is less porous than the portion of the same specimen obtained in the part other than said central portion.
However, with a sampler of the type illustrated in FIG. 1 (B), it is still difficult to prevent a diffusion of hydrogen up to the time when the final analytical data are obtained and experience has shown that the results are often lower than the expected values.
This invention has been accomplished in view of the above disadvantages of the prior art procedures.